Kent County Council Flood Investigation Report Location of Investigation: Snodland Date of incident: 10 June 2019 This document has been prepared by Kent County Council Flood and Water Management Team as the Lead Local Flood Authority under Section 19 of the Flood and Water Management Act 2010, with the assistance of: • Kent County Council • Tonbridge and Malling Borough Council • Kent Fire and Rescue Service The findings in this report are based on the information available to KCC at the time of preparing the report. KCC expressly disclaim responsibility for any error in or omission from this report. KCC does not accept any liability for the use of this report or its contents by any third party. This report can be found here where more information can be found about the requirements and trigger for a Section 19 investigation and the roles and responsibilities of Risk Management Authorities. For further information or to provide comments, please contact us at [email protected] Summary of event On the 10th June 2019 heavy rainfall was experienced across the country. 31 flood warnings and 1 flood alert were issued by the Environment Agency across England, with some areas of Kent recording in excess of 100mm of rain in a 24-hour period. The average rainfall for June in Kent is 53 mm1. Very heavy rainfall occurred county wide, with areas such as West Kingsdown, Vigo, Snodland and Swanley particularly badly affected. This report will focus on the Snodland area where around 20 properties are known to have been flooded by surface water, many of those were affected internally. The locations of the reported flood incidents are shown in Figure 1 and Table 1 provides a summary of the investigated flooding issues and known flood extent. 1 Source: https://www.southernwater.co.uk/water-for-life/regional-rainfall 1 www.kent.gov.uk Figure 1 Location of Reported Flood incidents in Snodland Table 1 Summary of the investigated flooding issues Location Details of flooding Source of Report Pout Road Internal flooding to 2 properties KCC Saltings Road Internal flooding to 2 properties, water was reported to have come KCC and TMBC from overflowing drain in Oxford Road and flowed into Saltings Road water reached a depth of 4 to 5 inches Recreation Avenue Internal flooding to 9 properties TMBC and KFRS Lakeside 6 properties affected TMBC, KFRS and Damage to vehicles due to depth of water KCC Pridmore Road 3 properties affected TMBC, KFRS and KCC High Street 1 property affected – water reached property no confirmation that KCC and TMBC water entered property garden was flooded Holborough Road 1 internal flooding to property water reached around 10 inches KFRS St Katherine Lane Internal Flooding to 1 property. Internal flooding at St Katherines KFRS School, 21 classrooms affected water reached a depth of 6 inches. School was closed for 2 days immediately after the flood. Roberts Road Highway Flooding KFRS Birling Road 1 property affected KFRS Rectory Close 1 property affected KFRS Cooper Road 1 property internally flooded KCC Sortmill Road Internal flooding to Retail Depot KCC Oxford Street Highway flooding KCC Freelands Road Highway flooding KCC 2 www.kent.gov.uk Site Location, Topography and Flood Risk Snodland is located north-west of Maidstone, situated close to the River Medway. The town sits at the base of the steep scarp slopes of the North Downs, small watercourses rise along the scarp slopes where the chalk gives way to gault clay and drain towards the Medway. Snodland predominantly sits over gault clay, however to the south at Ham Hill the lower elevation exposes Folkestone Beds Sandstone. Legend Open water course Culverted water course Figure 2 Location of the watercourse in Snodland A small water course rises in Stalks Wood to the west of Snodland, the water course flows in an easterly direction and passes through the former quarry site on Paddlesworth Road forming a large online lake. The catchment upstream of the lake covers an area of 0.7km2 and is predominantly arable farmland. The lake is owned by Tarmac Cement and Lime Ltd and forms part of the Paddlesworth Lake complex. The lake covers approximately 9 hectares with both the inlet and outlet located at the southern end of the lake. As The watercourse leaves the lake via a small channel where it enters a culvert on TMBC owned open greenspace. The culverted watercourse follows the road network underneath Ritch Road and St Benedict Road, before becoming open water at Roberts Road adjacent to Snodland C of E School, here the channel turns sharply to the east before passing under Pridmore Road. The open watercourse continues to run parallel to Roberts Road before entering a culvert at Birling Road. 3 www.kent.gov.uk Photo 1 Inlet to the culvert at Paddlesworth Lake Photo 2 Inlet to the culvert at Birling Road (27/06/2019) (18/06/2019) The culvert follows the route of Birling Road to the junction of Rectory Avenue and from here follows a straight line to the junction of Chapel Road and Malling Road. The culvert continues in an easterly direction along Rocfort Road before turning South to follow the route of the footpath which runs to the rear of Rectory Close and Saltings Road. After 200 meters the culvert turns east for a further 240 meters before becoming open water to the rear of properties on Saltings Road adjacent to the A228. At the A228 the watercourse enters a culvert and flows south becoming open ditch at the Ham Hill sewage works and finally joining the Leybourne Stream within the Country Park. The catchment area for this water course covers a total of 2.1km2. Photo 4 Security screen on outlet of pipe at Saltings Way Photo 3 Open channel between Salting Road and the (18/06/2019) A228 (18/06/2019) The Leybourne Stream flows from the south of Snodland, travelling on the eastern side of the A228, it travels through Leybourne Lakes country park. Turning north within the country park the stream travels adjacent to the eastern side of the sewage works and Sortmill Road industrial estate before turning east and passing underneath the railway line and joining the River Medway. 4 www.kent.gov.uk During rainfall events, water that is not captured by surface water drainage will follow the topography and naturally drain towards the watercourse. The updated Flood Map for Surface Water (uFMfSW) shown in Figure 3 is published on the Environment Agency website, it shows the areas around the watercourse are at high risk of surface water flooding, it should be noted that lakes and ponds are shown in dark blue on the map. The uFMfSW map follows the route of the watercourse and the locations of flooded properties correspond to this area. The national surface water flooding provided by the Environment Agency on the gov.uk long term flood risk information map2 indicates these areas to be at a medium to high risk of surface water flooding. Medium and high risk means that these areas have a chance of flooding of greater than 1% and 3.3% in any one year respectively. Figure 3 Environment Agency surface water flood map Drainage System Details Snodland is a densely built up area with a high percentage of impermeable surfaces, private drainage, roofs and highways. The town is drained by a combination of mechanisms, the older parts of the town drain to combined sewers, which carry both foul and surface water, and discharge at the sewage treatment works at Ham Hill near Snodland. Newer parts of the town are served by dedicated surface water sewers which discharge to a local watercourse. The drainage in some parts of the town is based on soakaways, which collect surface water from a small area and allow it to infiltrate into the ground. Soakaways rely on the natural permeability of the underlying rock for the water to soak into. They are usually made by digging large manholes that are lined with perforated concrete rings or open jointed brickwork. The manhole provides storage for water as it seeps through the perforations in the chamber into the surrounding bedrock over a period of 24 to 48 hours, ready to accept a subsequent storm. Prolonged very heavy rainfall or multiple heavy rainfall events within a short period can overwhelm the storage available in these soakaways and result in surface water flooding occurring. Individual soakaways serve relatively small areas of between two and six road gullies. 2 The flood warning information service long term flood risk maps are publicly accessible by visiting https://flood- warning-information.service.gov.uk/long-term-flood-risk/map 5 www.kent.gov.uk Figure 4. Southern Water public sewers within Snodland The public sewers in Snodland are owned and maintained by Southern Water. The drainage assets on the public highway are owned and maintained by KCC as the Highway Authority, the highway generally drains to the public sewer network, in some areas it drains to soakaways or via dedicated highway sewers, which are owned by KCC. At Pout Road, St Katherines Lane, Roberts Road and Lakeside the highway run-off is managed by the highway drainage network. The areas around St Benedicts Road including Freelands Road to the west, Pridmore Road, the High Street and Holborough Road to the north and Saltings Road and Rectory Close to the east are of more recent construction and have separate surface water and foul water drainage. The surface water sewers discharge to the culverted watercourse which is located underneath the highway. The area around Malling Road including Recreational Avenue and Oxford Street is within the older area of the town and is drained almost exclusively by combined sewers.